We describe experiments to study a recently discovered genetic locus in Drosophila with unprecedented features. The novel aspects of the yin-yang locus are that it houses two closely linked genes, one expressed only in the central nervous system of male animals (yang) and the other expressed only in the central nervous system of females (yin), the latter feature being deduced by the expression of a reporter gene. Continued structural studies will reveal the precise physical relationship of the two genes, the exon/intron configuration, and the sequence of the products. RNA in situ hybridization and immunohistochemistry will be performed to elucidate the pattern of expression of the two genes with special emphasis on CNS expression in both sexes. Additional mutants at the locus will be obtained to complement the existing mutation in the yang gene. The neuroanatomy of these mutants as well as their reproductive behavior will be studied to draw connections between sexually dimorphic gene expression and sexually dimorphic neuroanatomy and behavior. The biochemical function of each gene will be examined relative to the hypothesis that yin encodes a peptide transporter molecule and yang encodes a regulatory RNA confined in function to the nucleus. Finally, the regulation of the two genes will be studied using reporter gene constructs and existing mutants in the sex determination pathway. These experiments comprise a relatively comprehensive plan to dissect the novel aspects of yin and yang so that we can better understand the mechanisms of sex-limited gene expression and the relevance that this limitation has to sex-limited neuroanatomy and behavior.